Friday, April 22, 2011

Opposites Attract?

            A reader recently sent along a set of fantastic pictures of two snakes intertwined on a log overlooking a river. Apparently the e-mail has been circulating for a while; somewhere along the line it was suggested that these snakes were two Cottonmouths mating. Although these two snakes are mating, they’re not Cottonmouths, they are Brown Watersnakes, Nerodia taxispilota. The two species are often confused (a discussion of how they can be told apart is here).

Comparing the head of a Cottonmouth versus a non-venomous watersnake is a good way to tell the two snakes apart, but it’s difficult to make out the head of the big snake in the picture. In any case, the brown boxes on the body of the Brown Watersnake are distinctive.

What’s striking in these pictures is the giant size of the female snake, the male is only a fraction as big. Although they are both adult snakes, it’s hard to know it by looking at them. Female Brown Watersnakes can reach much larger sizes than males.  They are what we call sexually dimorphic, meaning that the size or shape of males and females differ. Humans are sexually dimorphic too, but this isn’t the website you’re looking for if you want pictures explaining how.

There are a few reasons why males and females of the same species might reach different sizes or come in different shapes. In the case of the Brown Watersnake we need to consider why there would be a benefit for females to get big, but not males.  One potential reason relates to how much energy is required to produce offspring. It doesn’t really matter how big a male is, his reproductive investment is always the same, but it takes a lot of energy to provide for a litter of snakes developing inside of you. For many reptiles, a bigger female can produce more babies. It’s easy to conceive how a big female will produce more babies that carry her genes for big body size, they grow up and produce more babies themselves, and so on.

We should ask why the males don’t get big too. Even if there is no real benefit, what would it hurt? Well, there is a cost associated with larger body sizes, you need to catch and eat more food to maintain a big body. For the females, this cost is outweighed by the benefit of producing more young. The males don’t have that benefit, so they stay small.

Another reason males and females differ is to limit competition. If all the individuals in a population eat one type of prey, and there is only so much of that prey type to go around, then the population can’t grow much and everyone is going to be fighting over food.  On the other hand, if half of the population (males) eat something different than the other half (females), then there is a lot more food for everybody.

One of the classic examples of this type of sexual dimorphism is clearly demonstrated among the map turtles. You might be hard pressed to recognize that the  turtle on the right is the same species as the one below, but they are both Barbour’s Map Turtles, Graptemys barbouri. This turtle can only be found in a few river drainages in Alabama, Georgia, and Florida, which is a big reason why they’re considered rare. The male turtle is the one with the regular-looking head on the right, they prefer to eat various soft-bodied insects and other invertebrates. The female is the one with the head that appears grotesquely large (although I assure you the male turtles don’t mind). Females have large heads to help them crack down on their preferred prey, clams and mussels. Females also reach larger sizes than the males, probably for the same reason that Brown Watersnakes do.

A third type of sexual dimorphism that we often observe is the exact opposite than what we see among the watersnakes. Sometimes, males get bigger than the females. When this type of variation occurs, it’s usually because males compete with each other for the chance to mate with a female. Snapping turtles do this, as do rattlesnakes. One of the most rare and exciting sights a nature-lover can see is two big male rattlesnakes reaching into the sky while they wrestle and push each other to the ground as their rattles buzz lazily. A recent acquaintance related to me she saw this happen once in the woods of northwestern Florida. She noted this was many years ago when she had a film camera and wouldn’t you know it, there was only one picture left in the roll.

The final kind of sexual dimorphism that we see in the animal kingdom (at least the last one I can think of) happens when males go to great lengths to impress females.  You’ve probably seen the little green lizards called Green Anoles, Anolis carolinensis. Maybe you’ve seen males doing push-ups while they flash their pink dewlaps (a collapsible fanlike structure underneath their chin). Females have dewlaps too, but they’re much smaller. Birds are the most famous participants of this strategy (think peacocks).

Lots of people ask me how to tell the difference between male and female snakes. The truth is, sometimes it’s hard to tell and you need to look for subtle differences in morphology (i.e. body shape). The length of the tail is a good way to tell them apart, but when I offer this hint I often get the same response, 

“I thought they were all tail”.


Thank you to the reader (who prefers to remain anonymous) who provided the Brown Watersnake pictures which inspired this blog. Barbour's Map Turtle pictures provided courtesy of Sean Sterrett.

Much of what I write is based on my experience in the field, however I also rely on the research of others, citations of some relevant scientific articles are below.

Shine, R. (1978). Sexual size dimorphism and male combat in snakes Oecologia, 33 (3), 269-277 DOI: 10.1007/BF00348113

Gibbons, J., & Lovich, J. (1990). Sexual Dimorphism in Turtles with Emphasis on the Slider Turtle (Trachemys scripta) Herpetological Monographs, 4 DOI: 10.2307/1466966

Tuesday, April 19, 2011

The Latest Rat Snake Freakout

Those of you who have following my blog for a while know that there are several recurring themes, (besides snakes, turtles, salamanders and other amphibians and reptiles in general). One common and popular topic relates to the tendency to grossly exaggerate the size of dead rattlesnakes. However, another topic that rarely rears its head (but is perhaps more darkly amusing) is when people absolutely and positively freak out when they encounter common native wildlife.

And, what kind of animal are people simultaneously unfamiliar with and terribly frightened by? Snakes of course.

Rat Snakes in particular inspire some incredible stories. This species (and I’m referring to Elaphe obsoleta AKA, more recently, Pantherophis alleghaniensis) ranges throughout the eastern and central United States and can be quite common in suitable habitats, which include hardwood forests and abandoned farms and barns. Rat snakes often can be often around these abandoned buildings because they are great places to find their favorite prey, which, you guessed it, tend to be small mammals like rats.

Although rat snakes can be found frequently in the southeastern United States (here I’m considering Black Rat Snakes and Gray Rat Snakes the same animal, although there is some recent genetic evidence suggesting we should treat them as separate species), their appearance often attracts a lot of commotion elsewhere. To be fair, they can be a large animal (the biggest specimens could reach seven or eight feet long) but they can hardly be considered exotic. Yet, when these snakes show up, perhaps crawling down from their usual tree hiding spots to find mates in the breeding season or food when they’re hungry, their arrival is generally not appreciated.

For example, when a Black Rat Snake was noticed in a Delaware neighborhood, local officials warned citizens to keep their pets and children under a close watch while the leviathan (which they claimed was likely an escaped python) was on the loose.

Less than a year later, we heard of an escaped mamba in Minnesota. The snake, which I’m sure you realize was a Black Rat Snake, was killed by frantic people who claim they feared for their lives.

This winter, we learned of a snake spotted in some Christmas decorations that made the mistake of revealing its presence to some individuals who weren’t filled with holiday cheer when greeting their guest.

And today, we learned of a boa constrictor or python that’s terrorizing a neighborhood in Illinois (caution: hyperlink leads to a picture some of you may find unpleasant). You can probably guess where I’m going with this.

Black Rat Snakes are common around southern Illinois, yet once again we hear of a monster snake from South America threatening pets and children.  We are provided with some pics of the culprit, which can be clearly identified by its color patterning. The picture was taken after the snake was stabbed with a board with a nail sticking out of it. Fortunately for the snake, it was able to escape. Unfortunately for the man who stabbed it, it is hard to verify his claim that it was 12 feet long.

A polite police chief checked out the area and, after finding a snake, diplomatically stated that, "The snake I saw was not a python, was not an anaconda, was not a boa constrictor”. The snake is a Black Rat Snake and hopefully it will not be spotted by anyone else in the neighborhood who panics at the sight of him.

Experts were consulted and they correctly identified the snake in the picture. Based on the article, one mentioned that even if the snake in question was a python there is no reason to fear for the neighborhood pets, because dogs and cats are too big for them to eat. After watching the embedded video, I believe she was misquoted (she was referring to Black Rat Snakes, not pythons, being too small to eat your dog).

Although it is possible that there are boa constrictors or pythons on the loose, the snake in the pictures (which is claimed to be the subject of the panic described here) is not one of them. Perhaps the residents of Bush, Illinois will take this opportunity to learn more about the fascinating wildlife in their own backyards.

A great resource for information about Illinois amphibians and reptiles can be found here, the site includes a straightforward way of identifying snakes here (remember not to handle animals you can't identify).

Sunday, April 10, 2011

Why are squirrels so stupid?

I imagine I’ve heard this question one hundred times if I’ve heard it once.

Perhaps this situation is familiar to you: sitting alongside the road, a squirrel’s big black eyes go wide as a car barrels down upon it. Suddenly, seemingly just moments before the vehicle passes the small mammal, it darts into the road. Knuckles clench the wheel and feet slam on the brakes as the imminent disaster approaches. So close that the squirrel’s tail likely brushes the rubber of the front tire, the animal reaches the other lane and apparent safety. All in the car exhale deeply as the crisis is averted.

Then, as relieved smiles are exchanged all around, the squirrel leaps into motion again. To everyone’s shock and horror, the squirrel doubles back to head back across the road. This time, the outcome isn’t as happy.

            The driver winces, maybe slaps the dashboard, and exclaims, “It ran right under the tire! There was nothing I can do. Why are squirrels so stupid!?”

            Given the squirrel’s seemingly suicidal behavior, it’s a fair question to ask. But, what’s the answer? How did the squirrel survive as a species if they’re always jumping in front of cars?

            I imagine that thousands of years ago, before we had roads, let alone cars barreling along them, the one thing a squirrel had to worry about, the one thought that pre-occupied their little squirrel brains more than anything else, was of being eaten by another animal.  And what type of animal were they most worried about? I would venture a guess and say birds of prey, like hawks.

            Hawks are efficient predators. One of their most effective hunting strategies involves sitting high in a tree overlooking an open area, like a field. Driving along the interstate, you may have noticed the occasional red-tailed hawk sitting on a powerline or exposed branch, intently peering down into the grassy road shoulder. They’re waiting and looking for their next meal. When a snake, small bird, or mammal ventures into this area, the sharp-eyed hawk flies into motion and quickly swoops down onto their hapless prey.

Imagine you’re a football player and you’re running full speed at someone else holding the ball, intent on tackling them as quickly as possible. If the person with a ball is standing still, your job is easy. But what if they suddenly shift ten feet to the left? You’ve got to quickly compensate, which slows you down as you recalibrate onto your target. Then, as soon as you’ve got a bead on them again, they shift to the right, heading back to where they came from. Your hips swivel in response to their movement, slowing you down some more and throwing you off balance.

            Squirrels use the same principle to avoid hawks and other birds of prey. If a squirrel notices a hawk swooping down, they’re going to try to zig and zag so the hawk has trouble zeroing in. If they do it right, the hawk is likely to miss the squirrel altogether and crash into the ground. In the meantime, the squirrel makes their escape into the dense brush or branches of a nearby tree.

            When a squirrel notices a truck barreling down the road, it doesn’t have time to figure out the make and model, it perceives a threat and it reacts. And its reaction is based on what’s helped them thwart their primary threat for thousands of years.  It’s not that squirrels are stupid; it is just that the behavior that’s worked for them for so long isn’t much help in avoiding new threats. Given enough time, we may find that some day in the future squirrels are beginning to run in straight lines. This may help them avoid cars, but it will also help hungry hawks. In any case, if this change in behavior is possible, it hasn’t happened yet. In the meantime, and if you’re interested in avoiding flattened squirrels, just slow down. It gives the squirrel more time to react and you’re less likely to have to swerve to miss them.


Sunday, April 3, 2011

Cottonmouth Myths IV: I Got Chased by a Cottonmouth

        Let’s get back to some common Cottonmouth myths…

            When I am introduced to new people, and the conversation inevitably turns to my presumably bizarre career choices, often I am asked whether a particular species of snake is aggressive. Occasionally, I am also asked to comment on what type of snake is the most aggressive. I don’t imagine my typical answers are very satisfying.

            I have never come across any species of snake, or any individual snake for that matter, that I would consider “aggressive.” I have, however, experienced a wide variety of defensive behaviors. Some snakes are prone to bite when handled, some play dead, some crawl away as quickly as possible, and some may even puke up their last meal in the hopes of convincing you they’re not worth eating (I’m convinced, I’m convinced).  None have initiated any contact with me, let alone in a manner that I would consider aggressive.

            Perhaps the snake with the reputation for being the most aggressive is, of course, the Cottonmouth. Legend has it that when they’re not falling into your boat, they are chasing you around the beach, eager to teach you a lesson for wandering into their territory.

            I have heard this tale enough to give it some credibility. However, the idea that Cottonmouths chase people can be chalked up to a misperception of snake behavior. First, let’s list the reasons why it might benefit a snake to initiate or prolong contact with a threatening figure perhaps one hundred times their size….Still thinking of one? Me too.

            Cottonmouth venom is potent, and a bite from any venomous snake warrants medical attention. But, a snakebite doesn’t typically result in immediate death, there is still plenty of time after a bite for an angry and envenomated person (or animal) to take out their wrath on the offending snake. And no snake is going to give up their life just to make a point. It’s a lose/lose situation.

            When confronted by a potential predator, an animal’s best bet is to head in the other direction as fast as possible. It doesn’t make sense to start fighting until it’s a last resort, there’s just too much at stake.

            Let’s break it down: in a fight with a predator, if the potential prey wins then it is still alive, just like it was before the encounter happened. If it loses, it’s dead meat. So, why engage in a risky behavior when the best-case scenario is you end up where you were when you started?

            In some cases, it does make sense for an animal to risk it all. Namely, when they’re protecting their offspring. A protective parent is not likely to take off running at the first sign of danger. Cottonmouths however, do not exhibit parental care. Once a female gives birth, her young are on their own. Perhaps Cottonmouths haven’t developed parental care since the venomous babies can do a good job of defending themselves without her help.

            But in any case, I mentioned earlier that I have heard multiple stories about Cottonmouths chasing people. Why would people think they’re being chased by these snakes?

            The answer is because if a snake feels threatened, it is going to try to get from point A to point B as quickly as possible. And if a person is in the way, it’s going to look like the snake is coming right for them. Cottonmouths feel safest in the water and if they are disturbed while they are traveling on land they are going to head straight into the lake. They will do this even if they have to go right through what disturbed them in the first place. Their behavior, it seems, is just hardwired this way.
            Keep them from getting into the water, and Cottonmouths are likely to exhibit some other defensive behaviors, such as coiling up and rapidly vibrating their tail (which may sound like a rattlesnake in dry leaves), or, by flashing their white mouths. Both behaviors are designed to scare off predators. Cottonmouths will sometimes do a lot, it seems, to avoid biting someone.
            One reader once asked me to comment on a story related to him by his friend. This man, fishing on the water’s edge one day, had had his afternoon interrupted by not one, but two angry Cottonmouths. These two snakes had been chasing each other through the water, interspersed with occasional wrestling bouts. After one of these wrestling matches, a snake had made a beeline for the wary fisherman, who promptly executed the animal. To his surprise, then the other snake headed right for him. This snake also met a similar fate.

            We can only guess what was motivating these animals. But here’s my take: Male Cottonmouths, like some other pit vipers, are known to engage in combat over females. To the untrained eye, they may appear as wrestling bouts. When a winner is decided, and we don’t exactly know how they figure this out, the victor is likely to head right to the female, in the hopes of breeding. Perhaps the female was hiding out somewhere on the other side of the fisherman. When the winning snake was dispatched, perhaps the other Cottonmouth saw an opportunity to mate after all.  Little did he know she was not worth it.

 Have you been 'chased' by a Cottonmouth? 


This isn't the first time I've written about Cottonmouths.  For a discussion of Cottonmouths allegedly dropping into boats, click here. Or, to read about breeding balls, click here. To learn about where in the world you can find Cottonmouths, and where you can't, click here.

I wrote about how these snakes are quick to show up when a lot of toads (= food) appear here.  I've also written about accompanying Cottonmouth researchers as they wade through swamps to catch snakes in the spring as the reptiles try to take advantage of the new warmth and at night in the summer as the venomous snakes swam around me.  Finally, I provide some tips on recognizing Cottonmouths from non-venomous watersnakes here.

Cottonmouth pictures provided courtesy of Fingerprince Prints.

Much of what I write is based on my experience in the field, however I also rely on the research of others, citations of some relevant scientific articles are below.

Gibbons, J., & Dorcas, M. (2002). Defensive Behavior of Cottonmouths (Agkistrodon piscivorus) toward Humans Copeia, 2002 (1), 195-198 DOI: 10.1643/0045-8511(2002)002[0195:DBOCAP]2.0.CO;2

D. B. Means (2010). Blocked-flight aggressive behavior in snakes. IRCF Reptiles & Amphibians, 17 (2), 76-78